DCs are uniquely suited for the efficient uptake and processing of antigens by endocytic and pinocytic pathways, and, upon maturation, the presentation of these antigens to CD4 and CD8 T cells. The contribution of ITAM/ITIM signaling pathways to DC function, either to promote T cell activation or anergy, will be investigated through the analysis of FcRs on DCs. The physiological ligand for these receptors, the immune complex, can function to modulate immune responses initiated by DCs either as a potent stimulator or inhibitor of APC function. We will test the hypothesis that perturbations of these balanced activation/inhibitory pathways on DCs can contribute to autoimmune responses through the following four Specific Aims: 1) Characterize the functional capabilities of ITAM and ITIM immunoreceptors on immature and mature DCs in signaling, antigen presentation and maturation in vitro. DC populations representing different maturation stages obtained from mice deficient in specific FcRs, signaling molecules or expressing defined FcR mutants will be used to examine the contribution of positive and negative signaling to DC function. Stimulation of these ex vivo derived cells by physiological ligands will address the physiological role of ITAM stimulation and ITIM inhibition on DC maturation and APC function; 2) Define the in vivo role of activation and inhibitory receptors in inducing T cell proliferation or anergy. Immune complexes of defined valency and FcR specificity will be generated to DC-expressed surface molecules to target either activation or inhibitory FcRs in FcR wild-type or mutant strains to address the role of ITAM cross-linking and ITIM modulation to T cell stimulation or anergy; 3) Evaluate the in vivo contribution of activation and inhibitory FcRs expressed on distinct cellular populations to T and B cell responses to immune complexes. This Specific Aim will dissect the contribution of activation and inhibitory FcRs to the IC mediated immune response by generating conditional deficiencies of these receptors on DCs by using DC restricted promoters in the Cre-lox system. The resulting animals will be challenged by defined ICs and the T cell or B cell response evaluated; and 4) Determine the contribution of activation and inhibitory FcRs to the maintenance of tolerance or susceptibility to autoimmunity in murine models of autoantibody responses. The contribution of inappropriate DC activation to the development of autoimmunity will be examined by characterizing DC function in autoimmune strains, by generating either B cell or DC conditional deficiencies of FcRIIB on B6, and by overexpressing RIIB on DCs or B cells to correct autoimmune phenotypes.